As the number of communication networks such as PSTN, IP networks or PBX increases and with the desire to be able to communicate between the different communication networks, a unit known as a media gateway unit has been developed that acts as a translation unit with communication (such as voice or data calls) from one communication network being connected to another communication network through the media gateway unit. The media gateway unit performs a conversion between the different transmission and coding techniques of the different communication networks. Thus, for example, a data call from a data modem in an IP network would connect to a data modem in the PSTN network through a media gateway unit and the media gateway unit converts the data signals between their state in the IP network and the state required for transmission over the PSTN network.
The media gateway unit can operate in a number of different modes depending on which networks are to be connected through the media gateway unit. For example, for a data call between an IP network and a PSTN network, the media gateway unit switches to a mode in which encoded data from the IP network is decoded and then sent to the PSTN network and vice versa. A voice connection can also be used for data transmission (e.g. between a modem, facsimile, etc.) and the data transmitted over a voice connection is known as voiceband data. The signal characteristics are however different for voice and voiceband data and transmitting them requires different approaches. The low bit-rate vocoders (e.g. G.723.1, G.729) commonly used for voice transmission in IP networks are designed and focused to handle voice signals and do not provide a reliable method for transmitting data. Several mechanisms are used to improve the reliability of the transmission of voiceband data (VBD) and one of the simplest, known as the VBD switch when the media gateway unit switches to a VBD mode, is to detect the voiceband data and to then bypass the vocoder processing. The mode of the media gateway unit is determined by signalling or set up information received at the media gateway unit such as PSTN signalling information, IP signalling information or modem or fax tones.
The switching of the media gateway unit to a mode may trigger certain events in the media gateway unit.
In communication networks supporting speech or voice communication, such as PSTN or IP, devices such as CODECs, echo suppressors and echo cancellers are used in the network to perform telephone processing functions for speech or voice signals. For example, echo cancellers are used to reduce and attempt to eliminate echoes formed when a speech signal from a user coupled to a local subscriber loop at one end traverses the PSTN to a user coupled to a local subscriber loop at another end. However, for data communication between data enabled devices (e.g. data modems), over a network which support voice communication (for example, one modem being part of a PSTN and another to an IP network), as the data signal is passed through the voice CODECs in the PSTN network, the data signal becomes distorted and the modems cannot communicate. Thus, when the media gateway unit determines from the signalling or set up information that a data call is to be made across a PSTN network, the media gateway unit switches mode to the VBD mode and on entering that mode an event is triggered to disable the voice processing devices in the PSTN network, such as the echo cancellers, so that the data signal is not distorted.
Similarly, for data communication through the media gateway unit and over an IP network, when the media gateway unit determines from the signalling or set up information that a data call is to be made across an IP network, the media gateway unit switches mode and on entering that mode an event is triggered to disable the IP vocoder which encodes the data according to the IP protocol so that uncoded data may be passed through the media gateway unit.
In order to detect that data communication has been initiated, the media gateway unit typically comprises a module which detects when a data communication has been initiated over a communication network, e.g. when a modem connects to a PSTN line and tries to communicate, and depending on the output of the module the media gateway unit is arranged to trigger certain events e.g. disable the echo cancellers and suppressors or IP vocoders. This is achieved by the module being arranged to detect a certain type of set up signal which is used to initiate data communication and which is typically known as an answer tone signal sent by the answering device. The answer tone signals ANS, ANSam are defined by ITU standards, ITU-T V.8 and ITU-T V.25, and in a basic form the answer tone signal is a single continuous tone with a frequency of 2100 Hz±15 Hz and a duration of 3.3±0.7 seconds. As described in these ITU standards, the answer tone may include 180° phase reversals at predetermined intervals, such as 425 to 475 ms, which is used by the media gateway unit to disable network echo cancellers.
U.S. Pat. No. 7,003,093 describes a tone detection processor which discriminates between tone and voice signals and determines the tones. The tone detection processor performs automatic gain control (AGC) to normalize the power of the tone or voice signal. Further, the energy of the tone or voice signals are determined at specific frequencies utilizing a Goertzel Filter process which implements a plurality of Goertzel filters. The tone detection processor determines whether or not a tone is present from the two maximum energy levels of the Goertzel filtered tone and if a tone exits, determines the type of tone. However, the tone detection method described in this patent is not very accurate since it only uses a Goertzel Filter process. This means that a high (approx 100%) Call Connect Reliability (CCR) for all standard types of telephone lines (EIA 1-7, ETSI1 and ETSI2) is not achievable. In addition, additional processing overhead is required by the use of AGC.
There is therefore a need to provide an improved method for detecting a set up signal for data communication which addresses at least some of the above referenced problems.